1. Field of the Invention
The present invention relates to a piezoelectric actuator, a liquid ejection head, an image forming apparatus, and a method of manufacturing a piezoelectric actuator, and more particularly, to a piezoelectric actuator comprising a piezoelectric body formed by a sputtering method.
2. Description of the Related Art
A piezoelectric actuator used in an inkjet head, or the like, is constituted, for example, by a diaphragm made of silicon (silicon substrate), a lower electrode formed on the upper surface of the diaphragm, a piezoelectric body (PZT film) formed on the upper surface of the lower electrode, and an upper electrode formed on the upper surface of the piezoelectric body. In an inkjet head which comprises a piezoelectric actuator of this kind, when an electric field is applied to the piezoelectric body which is sandwiched between the electrodes, the piezoelectric body deforms and the ink inside the pressure chamber is pressurized by the diaphragm, thereby causing an ink droplet to be ejected from a nozzle.
In a general piezoelectric actuator, it is possible to obtain the same amount of displacement regardless of the direction of the electric field applied to the piezoelectric body (in other words, whatever the direction in which the electric field is applied). Consequently, from the viewpoint of reducing costs and facilitating the wiring tasks in the drive IC (driver IC) which supplies a drive voltage to the piezoelectric actuator, it is common to adopt a composition which uses the lower electrode which is disposed on the diaphragm side (pressure chamber side) of the piezoelectric body as a common electrode (ground electrode) and uses the upper electrode which is disposed on the opposite side as the individual electrode (address electrode).
However, for example, Japanese Patent Application Publication No. 2004-79991 discloses a piezoelectric actuator which comprises a piezoelectric body (PZT film: lead zirconium titanate film) deposited by sputtering. In such a piezoelectric body, the piezoelectric body already has an established orientation direction (direction of polarization) when the film of piezoelectric body is deposited, and there is a difference in the amount of displacement depending on the orientation of the applied electric field. For example, FIG. 12 shows the relationship between the applied electric field and the displacement in the case of a PZT film deposited by sputtering. As shown in FIG. 12, when an electric field is applied in the same direction as the direction of polarization, then it is possible to obtain a positive amount of displacement which is directly proportional to the intensity of the electric field. On the other hand, if an electric field is applied in the opposite direction to the direction of polarization, then a negative amount of displacement which is directly proportional to the intensity of the electric field is obtained if the absolute value of the electric field intensity is small, but as the absolute value of the electric field intensity becomes larger, the amount of displacement changes from negative to positive. Therefore, in a piezoelectric actuator which comprises a piezoelectric body deposited by sputtering as described above, a desirable composition is one where the lower electrode is used as an individual electrode (address electrode) and the upper electrode is used as a common electrode (ground electrode), in order that an electric field is applied to the piezoelectric body in the same direction as the direction of orientation (direction of polarization) of the piezoelectric film. This is because, if the upper electrode is used as an individual electrode, then a negative voltage must be supplied to the upper electrode, and the costs relating to the driver IC and power source become several times greater than when a positive voltage is supplied.
Furthermore, if the diaphragm is constituted by a silicon substrate and the lower electrode is used as an individual electrode while the upper electrode is used as a common electrode, then a current leakage occurs between the plurality of lower electrodes (individual electrodes) via the diaphragm, and a problem of electrical cross-talk arises, which causes ink droplets to be ejected from nozzles where they are not supposed to be ejected, for example. Moreover, due to the increase in the electrostatic capacitance, there is also a drawback in that the power consumption increases.
On the other hand, in the piezoelectric actuator disclosed in Japanese Patent Application Publication No. 2006-6096, an insulating layer is disposed between the diaphragm and the lower electrode (individual electrode). However, in Japanese Patent Application Publication No. 2006-6096, the composition described above is used only in order to suppress the occurrence of wiring faults by eliminating step-differences in the wires, and it does not consider reversing the direction of polarization of the piezoelectric bodies deposited by sputtering. Therefore, it is difficult to resolve the problems of electrical cross-talk and increase in power consumption.
On the other hand, Japanese Patent Application Publication No. 10-286953 discloses a method of manufacturing an inkjet head by forming an electrode and a piezoelectric film on a substrate of monocrystalline magnesium oxide (MgO), and then depositing a thin film thereon to form a diaphragm, the resulting structure then being transfer (bonded) to pressure chambers formed in a silicon substrate or glass substrate. However, in a method of this kind, costs are high due to the use of magnesium oxide, and furthermore, it is difficult to align the piezoelectric bodies and the pressure chambers accurately in position, due to the fact that a transfer bonding method is used.